Keeps blowing fuses — alternator problem?

[PotvinV8]

I'm wondering if maybe my battery is just never charging up to full and so the alternator is happy to keep putting out 30A until the fuse burns out.

I would connect the alternator wire directly to the battery (bypass the fuse) and see if you still have problems. Your battery and charging system might be fine, but the alternator is "spiking" and killing your fuse. Alternators do funny things.

The fact that this happens at 50 mph makes me believe it may be rpm related and that the alternator isn't being "excited" until the motor is spun up, at speed, which then blows the fuse when the alternator "comes online". Some alternators have exciters built into them and they don't charge until spun up to a certain rpm. This can be bypassed, by the way. I had a roadster pickup where we mounted the alternator off the driveshaft and it wouldn't charge while driving around town, due to the lower rpms it was spinning. Bypassed the exciter and all was well.

Keep us posted, everyone loves a good mystery!

[eamonford]

Great suggestions from everyone. I’ll check for shorts in the places several have suggested, although if there is an intermittent short it may be difficult to detect under static conditions when I’m not driving.

Other updates: I had the battery tested today and it seems to be in great condition and also fully charged, so no problems there. I’ve already disconnected the headlights and horn, and even bypassed the ammeter and dash light to rule out any possible extra resistance from those components, so the only things left in the entire electrical system are the battery, alternator, ignition switch, coil and ballast resistor (both new), and points/condenser (also both new). Since I’ve observed the alternator putting out 20V with the engine at a high idle, I’m taking a guess that there’s a problem with the internal regulator, as others have suggested — my grandfather installed it at least 45 years ago, after all! I ordered a new alternator today, so when it arrives I will install it and see what happens next. Will report back here with the results!

[Patrick L.]

Last internal regulator I replaced was $12.

[katy]

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